Stable microbubble suspensions comprising saturated phospholipids for ultrasound echography

ABSTRACT

Disclosed are injectable suspensions of gas filled microbubbles in an aqueous carrier liquid usable as contrast agents in ultrasonic echography. The suspensions comprise amphipathic compounds of which at least one may be a laminarized phospholipid as a stabilizer of the microbubbles against collapse with time and pressure. The concentration of phospholipids in the carrier liquid is below 0.01% wt but is at least equal to or above that at which phospholipid molecules are present solely at the gas microbubble-liquid interface. Also disclosed is a method of preparation of the stable suspensions of air or gas filled microbubbles.

This is a Rule 60 Divisional of application Ser. No. 08/134,671, filed12 Oct. 1993 now U.S. Pat. No. 5,445,813.

TECHNICAL FIELD

The invention relates to injectable suspensions of gas filledmicrobubbles in an aqueous carrier comprising amphipathic compounds ofwhich at least one is a phospholipid stabilizer of the microbubblesagainst collapse with time and pressure. The phospholipid stabilizer maybe in a lamellar or laminar form. The invention also comprises a methodof making stable suspensions of microbubbles usable as contrast agentsin ultrasonic echography.

BACKGROUND OF INVENTION

Use of suspensions of gas microbubbles in a carrier liquid as efficientultrasound reflectors is well known in the art. The development ofmicrobubble suspensions as echopharmaceuticals for enhancement ofultrasound imaging followed early observations that rapid intravenousinjections can cause solubilized gases to come out of solution formingbubbles. Due to their substantial difference in acoustic impedancerelative to blood, these intravascular gas bubbles are found to beexcellent reflectors of ultrasound. Injecting into the blood-stream ofliving organisms suspensions of gas microbubbles in a carrier liquidstrongly reinforces ultrasonic echography imaging, thus enhancing thevisualisation of internal organs. Since imaging of organs and deepseated tissue can be crucial in establishing medical diagnosis a lot ofeffort is devoted to the development of stable suspensions of highlyconcentrated gas microbubbles which at the same time would be simple toprepare and administer, would contain a minimum of inactive species,would be capable of long storage and simple distribution. Many attemptstowards a solution which will satisfy these criteria have been made,however, none have provided a completely satisfactory result.

It has been known from EP-A-0 077 752 (Schering) that suspensions of gasmicrobubbles can be made by mixing an aqueous solution of a surfactantwith a solution of a viscosity enhancer as a stabilizer. The gas bubblesare introduced into the mixture by forcing the mixture of reagents andair through a small aperture. A suspension of CO₂ microbubbles may beobtained by addition of an acid to a mixture obtained from a solutioncontaining a surfactant and sodium bicarbonate and a solution of theviscosity enhancer. Mixing the components however, is to be carried outjust before use and the solution is to be consumed/injected immediatelyupon preparation. The disclosed surfactants (tensides) compriselecithins; esters and ethers of fatty acids and fatty alcohols withpolyoxyethylene and polyoxyethylated polyols like sorbitol, glycols andglycerol, cholesterol; and polyoxy-ethylene-polyoxypropylene polymers.Disclosed concentration of tensides in the suspension is between 0.01%and 10% wt and a preferred range is claimed to be between 0.5% to 5%.The viscosity enhancing and stabilizing compounds include for instancemono- and polysaccharides (glucose, lactose, sucrose, dextran,sorbitol); polyols, e.g. glycerol, polyglycols; and polypeptides likeproteins, gelatin, oxypolygelatin, plasma protein and the like. Thetotal amount of viscosity enhancing agent is limited to 0.5 and 50%. Useof polyoxypropylene-polyoxyethylene polymers (eg. Pluronic® F-68) asviscosity enhancing agent has also been disclosed. In the preferredexample, equivalent volumes of tenside, a 0.5% by weight aqueoussolution of Pluronic® F-68 (a polyoxypropylene-polyoxyethylenecopolymer), and the viscosity enhancer (a 10% lactose solution) arevigorously shaken together under sterile conditions to provide asuspension of microbubbles. The suspension obtained lasted over 2minutes and contained close to 50% of bubbles with a size below 50 μm.According to the document up to 50% of surfactants and/or viscosityenhancing agents may be employed, however, specific examples use between1% and 4% of Pluronic® F-68.

Easy-to-produce aqueous suspensions usable as imaging agents inultrasonic echography are disclosed in WO-91/15244 (Schneider et. al.).The suspensions contain film forming surfactants in laminar and/orlamellar form and, optionally, hydrophilic stabilizers. The laminarizedsurfactants can be in the form of liposomes i.e. microscopic vesicles,generally spherically shaped. These vesicles are usually formed of oneor more concentrically arranged bi-molecular layers of amphipathiccompounds i.e, compounds with a hydrophilic and a hydrophobic moieties.The molecules in the bilayers are organised so that the hydrophobicmoieties are in facing relationship, the hydrophilic moieties pointingtoward the water phase. The suspensions are obtained by exposing thelaminarized surfactants to air or a gas prior to or after admixing withan aqueous phase. Conversion of film forming surfactants into lamellarform is carried out according to various liposome forming techniquesincluding high pressure homogenisation or sonication under acoustic orultrasonic frequencies. The concentration of phospholipids claimed isbetween 0.01% and 20% and the concentration of microbubbles is between10⁸ and 10⁹ bubbles/ml. The microbubble suspensions remained stable formonths. The concentration of phospholipids in Example 1 is 0.5%.

An attempt toward a stable echogenic suspension is disclosed inWO-92/11873 (Beller et. al.). Aqueous preparations designed to absorband stabilise microbubbles for use as an echographic contrasting agentare made with polyoxyethylene/polyoxypropylene polymers and negativelycharged phospholipids such as phosphatidylglycerol,phosphatidylinositol, phosphatidylethanol-amine, phosphatidylserine aswell as their lysoforms. The concentration range of phospholipids in thepreparations may be between 0.01% and 5% by volume or weight, however,preparations with 1% of dipalmitoylphosphatidyl glycerol (DPPG) arespecifically disclosed and claimed. In addition to the negativelycharged phospholipids the compositions must contain between 0.1% and 10%of polymeric material (Pluronic® F-68). The total amount of solutes inthe preparations is between 5.1% and 10.4%. The concentration of themicrobubbles is not reported, however, according to the results given itmay be estimated to be about 10⁷ bubbles/ml. The stability of thesuspensions is reported to be better than that of EP-A-0 077 752.

Although the prior art compositions have merit, they still sufferseveral drawbacks which hamper their practical use. Firstly, some priorart compositions have relatively short life spans and secondly, theyhave a relatively low initial bubble count e.g. between 10⁴ and 10⁵bubbles/ml. This makes reproducibility and analysis of echographic testsmade with such compositions fairly difficult. In addition, sometechniques produce bubbles in a wide range of diameters (up to 50 μm)which prevents their use as echographic agents in certain applications(e.g. echography of the left heart).

The need for stable formulations of microbubbles which will resistpressure variations in the blood streams and have a good shelf life isfurther amplified by poor stability of some of the state-of-the-artcompositions. Microbubble formulations whose distribution and storagewould not present problems are particularly important.

Another drawback is that many of the heretofore known compositionscontain a high amount of different solutes such as polymers,phospholipids, electrolytes, and other which render their practical usemore and more difficult. For example, it is known that use ofpolyoxyethylene/polyoxypropylene polymers (Pluronic®) with particularpatients may cause unpleasant side effects (see for instance G. M.Vercellotti et. al. Blood (1982) 59, 1299). Preparations with a highphospholipid content in certain cases may also be undesirable. In anyevent, compositions with a high degree of various solutes areadministered reluctantly and their wide spread use is becomingconsidered to be undesirable. In fact, the trend in the pharmaceuticalindustry is to reduce concentrations of active and inactive ingredientsin various medical or pharmaceutical formulations to their lowestpossible levels and eliminate from the preparations everything that isnot necessary. Finding alternative methods and formulating moreeffective compositions continues to be important. This is particularlyso with microbubble suspensions used in echography since here theingredients have no curative effect and should lead to the leastpossible after consequences. However, as stated above, the state of theart preparations with typical concentrations in the range of 1% and 4%by weight and the teachings of prior art discourage use of reducedamounts of phospholipids and other non-phospholipid additives. Thereason for the discouragement is most probably hidden in the fact thatin the course of the routine experimentation further reduction inconcentration of the ingredients never produced suspensions which werestable enough to have any practical use or encourage further tinkeringin the lower end of the known range.

SUMMARY OF THE INVENTION

The present invention is based on the unexpected finding that verystable suspensions of a gas filled microbubbles comprising at least 10⁷microbubbles per milliliter may be obtained using phospholipids asstabilizers even if very low concentrations thereof are employed. Thesuspensions usable as contrasting agents in ultrasonic echography areobtained by suspending in an aqueous carrier at least one phospholipidas a stabiliser of the microbubbles against collapse with time andpressure, the concentration of the phospholipids being below 0.01% wt.but equal to or higher than that at which the phospholipid molecules arepresent solely at the gas microbubble-liquid interface.

It was quite unexpected to discover that as negligible amounts of thephospholipid surfactants involved here (used alone or with a relativelysmall proportions of other amphiphiles) can so effectively stabilizemicrobubbles. It is postulated that, in the presence of otheramphipathic compounds (such as Pluronic®) the mutual cohesion betweenstabilizer molecules is decreased and formation of monomolecularphospholipid films is inhibited. However, in the absence of largeamounts of other amphiphilic agents, the unhindered intermolecularbinding forces (electrostatic interaction or hydrogen bonding) betweenphospholipid molecules are sufficient to ensure formation of stablefilm-like structures stabilizing the bubbles against collapse orcoalescence.

According to the invention, suspensions of high microbubbleconcentration, high stability, long storage capacity and ease ofpreparation may be obtained even if the concentrations of surfactantsand other additives in the suspensions are kept well below the levelsused in the state-of-the-art formulations. The amount of phospholipidsused in the compositions of the invention may be as low as about thatonly necessary for formation of a single monolayer of the surfactantaround the gas microbubbles while the concentration of the bubbles inthe suspension is maintained above 10⁷ microbubbles per milliliter. Inthe present invention, microbubbles with a liposome-like double layer ofsurfactant (gas filled liposomes) are not likely to exist and have notbeen observed.

Suspensions with high microbubble concentrations e.g. between 10⁹ and10¹⁰ bubbles/ml of relatively high stability and long storage capacitymay be prepared even if the concentration of the phospholipidsurfactants are kept well below the levels known in the art. Suspensionswith as little as 1 μg of phospholipids per ml may be prepared as longas the amount of the surfactants used is not below that which isnecessary for formation of a single monolayer of the lipids around thegas microbubbles and as long as they are produced according to one ofthe methods herein disclosed.

Calculations have shown that for bubble concentrations of 10⁸ bubbles/mldepending on the size distribution of the microbubbles thisconcentration may be as low as 1 μg/ml or 0.0001%, however, thephospholipid concentrations between 0.0002% and up to 0.01% arepreferred. More preferably the concentration of the phospholipids in thestable suspensions of microbubbles of the invention is between 0.001%and 0.009%. Although further reduction of the amount of phospholipids inthe suspension is possible, suspensions prepared with less than 0.0001%wt. are unstable, their total bubble count is low and their echographicresponse upon injection is not satisfactory. On the other hand,suspensions prepared with more than 0.01% of phospholipids uponinjection do not perform better i.e. their stability and echographicresponse do not further improve with the concentration. Thus, the higherconcentrations may only increase the probability of undesirable sideeffects as set out in the discussion of the prior art. It is tentativelypostulated that only the segments of the surfactants which are in thelamellar or laminar form can effectively release molecules organizedproperly to stabilize the bubbles. This may explain why theconcentration of the surfactant may be so low without impairing thestability of the gas bubbles.

The suspensions of the invention offer important advantages over thecompositions of the prior art not only because of the low phospholipidcontent but also because the total amount of injected solutes i.e.lipids and/or synthetic polymers and other additives is between 1,000and 50,000 times lower than heretofore. This is achieved without anyloss of microbubble concentration i.e. echogenicity or stability of theproduct. In addition to the very low concentration of solutes, theinvention provides suspensions which may contain only the microbubbleswhose contribution to the echographic signal is relatively significanti.e. suspensions which are free of any microbubbles which do notactively participate in the imaging process.

Needless to say that with such low concentrations of solutes in theinjectable composition of the invention probability of undesirable sideeffects is greatly reduced and elimination of the injected agent issignificantly improved.

The microbubble suspensions with low phospholipid content of theinvention may be prepared from the film forming phospholipids whosestructure has been modified in a convenient manner e.g. by freeze-dryingor spray-drying solutions of the crude phospholipids in a suitablesolvent. Prior to formation of the suspension by dispersion in anaqueous carrier the freeze dried or spray dried phospholipid powders arecontacted with air or another gas. When contacted with the aqueouscarrier the powdered phospholipids whose structure has been disruptedwill form lamellarized or laminarized segments which will stabilise themicrobubbles of the gas dispersed therein. Conveniently, the suspensionswith low phospholipid content of the invention may also be prepared withphospholipids which were lamellarized or laminarized prior to theircontacting with air or another gas. Hence, contacting the phospholipidswith air or another gas may be carried out when the phospholipids are ina dry powder form or in the form of a dispersion of laminarizedphospholipids in the aqueous carrier.

The term lamellar or laminar form indicates that the surfactants are inthe form of thin films or sheets involving one or more molecular layers.In this form, the surfactant molecules organize in structures similar tothat existing in liposome vesicles. As described in WO-A-91/15244conversion of film forming surfactants into lamellar form can easily bedone by any liposome forming method for instance by high pressurehomogenisation or by sonication under acoustical or ultrasonicfrequencies. The conversion into lamellar form may also be performed bycoating microparticles (10 μm or less) of a hydrosoluble carrier solid(NaCl, sucrose, lactose or other carbohydrates) with a phospholipid withsubsequent dissolution of the coated carrier in an aqueous phase.Similarly, insoluble particles, e.g. glass or resin microbeads may becoated by moistening in a solution of a phospholipid in an organicsolvent following by evaporation of the solvent. The lipid coatedmicrobeads are thereafter contacted with an aqueous carrier phase,whereby liposomic vesicles will form in the carrier phase. Also,phospholipids can be lamellarized by heating slightly above criticaltemperature (Tc) and gentle stirring. The critical temperature is thetemperature of gel-to-liquid transition of the phospholipids.

Practically, to produce the low phospholipid content suspensions ofmicrobubbles according to the invention, one may start with liposomesuspensions or solutions prepared by any known technique as long as theliposomic vesicles are "unloaded", i.e. they do not have encapsulatedtherein any foreign material but the aqueous phase of the solutionitself.

The introduction of air or gas into a liposome solution can be effectedby usual means, injection i.e. forcing air or gas through tiny orificesinto the liposome solution, or simply dissolving the gas in the solutionby applying pressure and then suddenly releasing the pressure. Anotherway is to agitate or sonicate the liposome solution in the presence ofair or another physiologically acceptable gas. Also one can generate theformation of a gas within the solution of liposomes itself, for instanceby a gas releasing chemical reaction, e.g. decomposing a dissolvedcarbonate or bicarbonate by acid.

When laminarized surfactants are suspended in an aqueous liquid carrierand air or another gas is introduced to provide microbubbles, it isthought that the microbubbles become progressively surrounded andstabilised by a monomolecular layer of surfactant molecules and not abilayer as in the case of liposome vesicles. This structuralrearrangement of the surfactant molecules can be activated mechanically(agitation) or thermally. The required energy is lower in the presenceof cohesion releasing agents, such as Pluronic®. On the other hand,presence of the cohesion releasing agents in the microbubbleformulations reduces the natural affinity between phospholipid moleculeshaving as a direct consequence a reduced stability of the microbubblesto external pressures (e.g. above 20-30 Torr).

As already mentioned, to prepare the low phospholipid contentsuspensions of the invention, in place of phospholipid solutions, onemay start with dry phospholipids which may or may not be lamellarized.When lamellarized, such phospholipids can be obtained for instance bydehydrating liposomes, i.e. liposomes which have been prepared normallyby means of conventional techniques in the form of aqueous solutions andthereafter dehydrated by usual means. One of the methods for dehydratingliposomes is freeze-drying (lyophilization), i.e. the liposome solution,preferably containing hydrophilic compounds, is frozen and dried byevaporation (sublimation) under reduced pressure.

In another approach, non-lamellarized or non-laminarized phospholipidsmay be obtained by dissolving the phospholipid in an organic solvent anddrying the solution without going through liposome formation. In otherwords, this can be done by dissolving the phospholipids in a suitableorganic solvent together with a hydrophilic stabiliser substance e.g. apolymer like PVP, PVA, PEG, etc. or a compound soluble both in theorganic solvent and water and freeze-drying or spray-drying thesolution. Further examples of the hydrophilic stabiliser compoundssoluble in water and the organic solvent are malic acid, glycolic acid,maltol and the like. Any suitable organic solvent may be used as long asits boiling point is sufficiently low and its melting point issufficiently high to facilitate subsequent drying. Typical organicsolvents would be for instance dioxane, cyclohexanol, tertiary butanol,tetrachlorodifluoro ethylene (C₂ Cl₄ F₂) or 2-methyl-2-butanol however,tertiary butanol, 2-methyl-2-butanol and C₂ Cl₄ F₂ are preferred. Inthis variant the criteria used for selection of the hydrophilicstabiliser is its solubility in the organic solvent of choice. Thesuspensions of microbubbles are produced from such powders using thesame steps as with powders of the laminarized phospholipids.

Similarly, prior to effecting the freeze-drying of pre-lamellarized orpre-laminarized phospholipid solutions, a hydrophilic stabilisercompound is dissolved in the solution. However, here the choice of thehydrophilic stabilisers is much greater since a carbohydrate likelactose or sucrose as well as a hydrophilic polymer like dextran,starch, PVP, PVA, PEG and the like may be used. This is useful in thepresent invention since such hydrophilic compounds also aid inhomogenising the microbubbles size distribution and enhance stabilityunder storage. Actually making very dilute aqueous solutions(0.0001-0.01% by weight) of freeze-dried phospholipids stabilised with,for instance, a 10:1 to 1000:1 weight ratio of polyethyleneglycol tolipid enables to produce aqueous microbubbles suspensions counting 10⁹⁻10¹⁰ bubbles/ml (size distribution mainly 0.5-10 μm) which are stable,without significant observable change, even when stored for prolongedperiods. This is obtained by simple dissolution of the air-stored driedlaminarized phospholipids without shaking or any violent agitation. Thefreeze-drying technique under reduced pressure is very useful because itpermits, restoration of the pressure above the dried powders with anyphysiologically acceptable gas, i.e. nitrogen, CO₂, argon, methane,freons, SF₆, CF₄, etc., whereby after redispersion of the phospholipidsprocessed under such conditions suspensions of microbubbles containingthe above gases are obtained.

It has been found that the surfactants which are convenient in thisinvention can be selected from amphipathic compounds capable of formingstable films in the presence of water and gases. The preferredsurfactants include the lecithins (phosphatidylcholine) and otherphospholipids, inter alia phosphatidic acid (PA), phosphatidylinositolphosphatidyl-ethanolamine (PE), phosphatidylserine (PS),phosphatidylglycerol (PG), cardiolipin (CL), sphingomyelins. Examples ofsuitable phospholipids are natural or synthetic lecithins, such as eggor soya bean lecithin, or saturated synthetic lecithins, such as,dimyristoylphosphatidylcholine. dipalmitoylphosphatidylcholine,distearoylphosphatidylcholine or diarachidoylphosphatidylcholine orunsaturated synthetic lecithins, such as dioleylphosphatidyl choline ordilinoleylphosphatidylcholine, with saturated lecithins being preferred.

Additives like cholesterol and other substances can be added to one ormore of the foregoing lipids in proportions ranging from zero to 50% byweight. Such additives may include other non-phospholipid surfactantsthat can be used in admixture with the film forming surfactants and mostof which are known. For instance, compounds like polyoxypropylene glycoland polyoxyethylene glycol as well as various copolymers thereof,phosphatidylglycerol, phosphatidic acid, dicetylphosphate, fatty acids,ergosterol, phytosterol, sitosterol, lanosterol, tocopherol, propylgallate, ascorbyl palmitate and butylated hydroxy-toluene. The amount ofthese non-film forming surfactants are usually up to 50% by weight ofthe total amount of surfactants but preferably between 0 and 30%. Againthis means that the concentration of the various additives in the lowphospholipid content suspensions of the invention are in the range of0-0.05% which is more than one hundred times less than in thecompositions known so far.

It should also be mentioned that another feature of the suspensions ofthe invention is a relatively "high" gas entrapping capacity of themicrobubbles i.e. high ratio between the amount of the surfactant andthe total amount of the entrapped gas. Hence, with suspensions in whichthe microbubbles have sizes in the 1 to 5 μm range, it is tentativelyestimated that the weight ratio of phospholipids present at the gasbubble-liquid interface to the volume of entrapped gas under standardconditions is between 0.1 mg/ml and 100 mg/ml.

In practice all injectable compositions should also be as far aspossible isotonic with blood. Hence, before injection, small amounts ofisotonic agents may also be added to the suspensions of the invention.The isotonic agents are physiological solutions commonly used inmedicine and they comprise aqueous saline solution (0.9% NaCl), 2,6%glycerol solution, 5% dextrose solution, etc.

The invention further concerns a method of making stable suspensions ofmicrobubbles according to claim 1 usable as contrast agents inultrasonic echography. Basically, the method comprises adapting theconcentration of the phospholipids in the suspension of microbubblesstabilized by said phospholipids to a selected value within the limitsset forth in the claims. Usually, one will start with a microbubblesuspension containing more phospholipids than the value desired and onewill reduce the amount of said phospholipids relatively to the volume ofgas or air entrapped in the microbubble, without substantially reducingthe count of echogenerating bubbles. This can be done, for instance, byremoving portions of the carrier liquid containing phospholipids notdirectly involved at the air/liquid interface and diluting thesuspension with more fresh carrier liquid. For doing this, one maycreate within the suspension region (a) where the echogenerating bubbleswill gather and region (b) where said bubbles are strongly diluted. Thenthe liquid in region (b) can be withdrawn by separation by usual means(decantation, siphoning, etc.) and a comparable volume of fresh carrierliquid is supplied for replenishment to the suspension. This operationcan be repeated one or more times, whereby the content in phospholipidsnot directly involved in stabilizing the bubbles will be progressivelyreduced.

It is generally not desirable to achieve complete removal of thephospholipid molecules not present at the bubble gas/liquid interface assome unbalance from equilibrium may result, i.e. if the depletion isadvanced too far, some surfactant molecules at the gas/liquid interfacemay be set free with consequent bubble destabilization. Experiments haveshown that the concentration of phospholipids in the carrier liquid maybe decreased down to within the neighborhood of the lower limit setforth in the claims without significant changes in properties andadverse effects. This means that, actually, the optimal phospholipidconcentration (within the given limits) will be rather dictated by thetype of application i.e. if relatively high phospholipid concentrationsare admissible, the ideal concentration value will be near the upperlimit of the range. On the other hand, if depending on the condition ofthe patient to be diagnosed, the absolute value of phospholipids must befurther reduced, this can be done without adverse effects regardingmicrobubble count and echogenic efficiency.

An embodiment of the method comprises selecting a film formingsurfactant and optionally converting it into lamellar form using one ofthe methods known in the art or disclosed hereinbefore. The surfactantis then contacted with air or another gas and admixed with an aqueousliquid carrier in a closed container whereby a suspension ofmicrobubbles will form. The suspension is allowed to stand for a whileand a layer of gas filled microbubbles formed is left to rise to the topof the container. The lower part of the mother liquor is then removedand the supernatant layer of microbubbles washed with an aqueoussolution saturated with the gas used in preparation of the microbubbles.This washing can be repeated several times until substantially allunused or free surfactant molecules are removed. Unused or freemolecules means all surfactant molecules that do not participate information of the stabilising monomolecular layer around the gasmicrobubbles.

In addition to providing the low phospholipid content suspensions, thewashing technique offers an additional advantage in that it allowsfurther purification of the suspensions of the invention, i.e. byremoval of all or almost all microbubbles whose contribution to theechographic response of the injected suspension is relativelyinsignificant. The purification thus provides suspensions comprisingonly positively selected microbubbles, i.e. the microbubbles which uponinjection will participate equally in the reflection of echographicsignals. This leads to suspensions containing not only a very lowconcentration of phospholipids and other additives, but free from anymicrobubbles which do not actively participate in the imaging process.

In a variant of the method, the surfactant which optionally may be inlamellar form, is admixed with the aqueous liquid carrier prior tocontacting with air or another gas.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is graphical presentation of echographic responses as a functionof the microbubble concentration for a freshly prepared suspensionaccording to the invention.

Suspensions and the method of making low phospholipid contentsuspensions of the invention will be further illustrated by thefollowing examples:

EXAMPLE 1

Multilamellar vesicles (MLVs) were prepared by dissolving 240 mg ofdiarachidoylphosphatidylcholine (DAPC, from Avanti Polar Lipids) and 10mg of dipalmitoyl-phosphatidic acid (DPPA acid form, from Avanti PolarLipids) in 50 ml of hexane/ethanol (8/2, v/v) then evaporating thesolvents to dryness in a round-bottomed flask using a rotary evaporator.The residual lipid film was dried in a vacuum dessicator. After additionof water (5 ml), the suspension was incubated at 90° C. for 30 minutesunder agitation. The resulting MLVs were extruded at 85° C. through a0.8 μm polycarbonate filter (Nuclepore®). 2.6 ml of the resulting MLVpreparation were added to 47.4 ml of a 167 mg/ml solution of dextran10'000 MW (Fluka) in water. The resulting solution was thoroughly mixed,transferred in a 500 ml round-bottom flask, frozen at -45° C. andlyophilised under 0.1 Torr. Complete sublimation of the ice was obtainedovernight. Thereafter, air pressure was restored in the evacuatedcontainer. Various amounts of the resulting powder were introduced inglass vials (see table) and the vials were closed with rubber stoppers.Vacuum was applied via a needle through the stopper and the air removedfrom vials. Upon evacuation of air the powder was exposed to sulfurhexafluoride gas SF₆.

Bubble suspensions were obtained by injecting in each vial 10 ml of a 3%glycerol solution in water (through the stopper) followed by gentlemixing. The resulting microbubble suspensions were counted using ahemacytometer. The mean bubble size (in volume) was 2.2 μm.

    ______________________________________                                        Dry weight   Phospholipid conc.                                                                         Concentration                                       (mg/ml)      (μg per ml)                                                                             (bubbles/ml)                                        ______________________________________                                        0.5          8            9.0 × 10.sup.6                                1            16           1.3 × 10.sup.7                                5            81           7.0 × 10.sup.7                                10           161          1.4 × 10.sup.8                                ______________________________________                                    

Preparations were injected to rabbits (via the jugular vein) as well asminipigs (via the ear vein) at a dose of 1 ml/5 kg. In vivo echographicmeasurements were performed using an Acuson XP128 ultrasound system(Acuson Corp. USA) and a 7 MHz sector transducer. The animals wereanaesthetised and the transducer was positioned and then fixed in placeon the left side of the chest providing a view of the right and leftventricles of the heart in the case of rabbit and a longitudinalfour-chamber view in the case of the minipig. The preparation containing0.5 mg/ml dry weight gave slight opacification of the right as well asthe left ventricle in rabbits and in minipigs. The opacification,however, was superior with the 1, 5 and 10 mg/ml preparations.

EXAMPLE 2

Lyophilisates were prepared as described in Example 1 with air (insteadof SF₆) in the gas phase. The lyophilisates were then suspended in 0.9%saline (instead of a 3% glycerol solution). Similar bubbleconcentrations were obtained. However, after injection in the rabbit orthe minipig the persistence of the effect was shorter e.g. 10-20 sinstead of 120 s. Moreover, in the minipig the opacification of the leftventricle was poor even with the 10 mg/ml preparation.

EXAMPLE 3

MLV liposomes were prepared as described in Example 1 using 240 mg ofDAPC and 10 mg of DPPA (molar ratio 95:5). Two milliliters of thispreparation were added to 20 ml of a polyethyleneglycol (PEG 2'000)solution (82.5 mg/ml). After mixing for 10 min at room temperature, theresulting solution was frozen during 5 min at -45° C. and lyophilisedduring 5 hours at 0.2 mbar. The powder obtained (1.6 g) was transferredinto a glass vial equipped with a rubber stopper. The powder was exposedto SF₆ (as described in Example 1) and then dissolved in 20 ml ofdistilled water. The suspension obtained showed a bubble concentrationof 5×10⁹ bubbles per ml with a median diameter in volume of 5.5 μm. Thissuspension was introduced into a 20 ml syringe, the syringe was closedand left in the horizontal position for 24 hours. A white layer ofbubbles could be seen on the top of solution in the syringe. Most of theliquid phase (˜16-18 ml) was evacuated while the syringe was maintainedin the horizontal position and an equivalent volume of fresh, SF₆-saturated, water was introduced. The syringe was then shaken for awhile in order to homogenise the bubbles in the aqueous phase. A seconddecantation was performed under the same conditions after 8 hoursfollowed by three further decantations performed in four hour intervals.The final bubble phase (batch P145) was suspended in 3 ml of distilledwater. It contained 1.8×10⁹ bubbles per ml with a median diameter involume of 6.2 μm. An aliquot of this suspension (2 ml) was lyophilisedduring 6 hours at 0.2 mbar. The resulting powder was dissolved in 0.2 mlof tetrahydrofuran/water (9/1 v/v) and the phospholipids present in thissolution were analysed by HPLC using a light scattering detector. Thissolution contained 0.7 mg DAPC per ml thus corresponding to 3.9 μg ofphospholipids per 10⁸ bubbles. A Coulter counter analysis of the actualbubble size distribution in batch P145 gave a total surface of 4,6×10⁷μm² per 10⁸ bubbles. Assuming that one molecule of DAPC will occupy asurface of 50Å², one can calculate that 1,3 μg of DAPC per 10⁸ bubbleswould be necessary to form a monolayer of phospholipids around eachbubble. The suspension P145 was than left at 4° C. and the concentrationof gas bubbles measured on a regular basis. After 10 days, the productlooked as good as after its preparation and still contained 1-1.2×10⁹bubbles per ml. The exceptional stability was found very surprisingconsidering the extremely low amount of phospholipids in the suspension.

The experiment described above was repeated on a second batch ofmicrobubbles using a shorter decantation time in order to collectpreferably larger bubbles (batch P132). The median diameter in volumeobtained was 8.8 μm and the total surface determined with the Coultercounter was 22×10⁸ μm² per 10⁸ bubbles. The calculation showed that 6 μgDAPC for 10⁸ bubbles would be necessary to cover this bubble populationwith a monolayer of DAPC. The actual amount of DAPC determined by HPLCwas 20 μg per 10⁸ bubbles. Taking into account the difficulty ofobtaining precise estimates of the total surface of the bubblepopulation, it appears that within the experimental error, the resultsobtained are consistent with coverage of the microbubbles with onephospholipid layer.

Echographic measurements performed with different washed bubblepreparations showed that upon separation the lower phase gives a muchweaker echographic signal than the upper phase or a freshly preparedsample. On a first sight this seemed normal as the white layer on thetop of the syringe contained the majority of the gas microbubblesanyway. However, as shown in FIG. 1 the bubble count showed asurprisingly high microbubble population in the lower layer too. Onlyupon Coulter measurement it became apparent that the microbubbles had asize below 0.5 μm, which indicates that with small bubbles even when inhigh concentration, there is no adequate reflection of the ultrasoundsignal.

A four fold dilution of the preparation P132 in a 3% glycerol solutionwas injected in the minipig (0.2 ml/kg). The preparation of washedbubbles containing 2.5×10⁷ bubbles per ml and 5 μg of phospholipids perml provided excellent opacification in the left and right ventricle withoutstanding endocardial border delineation. Good opacification was alsoobtained by injecting to a minipig an aliquot of preparation P145(diluted in 3% glycerol) corresponding to 0.2 μg of phospholipids perkg. Contrast was even detectable in the left ventricle after injectionof 0.02 μg/kg. Furthermore, in the renal artery the existence of acontrast effect could be detected by pulsed Doppler at phospholipiddoses as low as 0.005 μg/kg.

It follows that as long as the laminarized phospholipids are arranged ina single monolayer around the gas microbubbles the suspensions producedwill have adequate stability. Thus providing an explanation for thepresent unexpected finding and demonstrating that the amount ofphospholipids does not have to be greater than that required forformation of a monolayer around the microbubbles present in thesuspension.

EXAMPLE 4

A solution containing 48 mg of DAPC and 2 mg of DPPA in hexane/ethanol8/2 (v/v) was prepared and the solvent evaporated to dryness (asdescribed in Example 1). 5 mg of the resulting powder and 375 mg ofpolyethyleneglycol were dissolved in 5 g of tert-butanol at 60° C. Theclear solution was then rapidly cooled to -45° C. and lyophilised. 80 mgof the lyophilisate was introduced in a glass vial and the powderexposed to SF₆ (see Example 1). A 3% glycerol solution (10 ml) was thenintroduced in the vial and the lyophilisate dissolved by gentleswirling. The resulting suspension had 1.5×10⁸ bubbles per ml with amedian diameter (in volume) of 9.5 μm. This solution was injected to arabbit providing outstanding views of the right and left ventricle. Evena ten fold dilution of this suspension showed strong contrastenhancement.

EXAMPLE 5

The procedure of Example 4 was repeated except that the initialdissolution of the phospholipids in hexane/ethanol solution was omitted.In other words, crude phospholipids were dissolved, together withpolyethylene glycol in tertiary butanol and the solution wasfreeze-dried; thereafter, the residue was suspended in water. Severalphospholipids and combinations of phospholipids with other lipids wereinvestigated in these experiments. In the results shown in the nexttable the phospholipids were dissolved in a tertiary butanol solutioncontaining 100 mg/ml of PEG 2'000. The residues obtained after freezedrying were saturated with SF₆ (see Example 1), then dissolved indistilled water at a concentration of 100 mg dry weight per ml.

    ______________________________________                                                      Conc. in tert-                                                  Lipid mixture butanol   Bubble conc.                                                                            Median diam.                                (weight ratio)                                                                              (mg/ml)   (×10.sup.9 /ml)                                                                   (μm)                                     ______________________________________                                        DSPC          2         1.3       10                                          DAPC/DPPG (100/4)                                                                           2         3.8       7                                           DSPC/Chol (2/1)                                                                             6         0.1       40                                          DAPC/Plur F68 (2/1)                                                                         6         0.9       15                                          DAPC/Palm. ac. (60/1)                                                                       2         0.6       11                                          DAPC/DPPA (100/4)                                                                           1         2.6       8                                           DAPC/Chol/DPPA (8/1/1)                                                                      8         1.2       19                                          DAPC/DPPA (100/4)*                                                                          5         2.4       18                                          ______________________________________                                         Legend                                                                        DAPC = diarachidoylphosphatidyl choline                                       DSPC = distearoylphosphatidyl choline                                         DPPG = dipalmitoylphosphatidyl glycerol (acid form)                           DPPA = dipalmitoylphosphatidic acid                                           Chol = cholesterol                                                            Palm. ac. = palmitic acid                                                     Plur F68 = Pluronic ® F68                                                 *In this experiment, CF.sub.4 was used as gas instead of SF.sub.6        

In all cases the suspensions obtained showed high microbubbleconcentrations indicating that the initial conversion of phospholipidsinto liposomes was not necessary. These suspensions were diluted in0.15M NaCl and injected to minipigs as described in Example 3. In allcases outstanding opacification of the right and left ventricles as wellas good delineation of the endocardial border were obtained at doses of10-50 μg of lipids per kg body weight or less.

EXAMPLE 6

PEG-2000 (2 g), DAPC (9.6 mg) and DPPA (0.4 mg) were dissolved in 20 mlof tertiary butanol and the solution was freeze dried overnight at 0.2mbar. The powder obtained was exposed to SF₆ and then dissolved in 20 mlof distilled water. The suspension containing 1.4×10⁹ bubbles per ml (asdetermined by hemacytometry) was introduced into a 20 ml syringe, whichwas closed and left in horizontal position for 16 hours. A white layerof bubbles could be seen on top of the solution. The lower phase (16-18ml) was discarded while maintaining the syringe horizontally. Anequivalent volume of fresh SF₆ -saturated distilled water was aspiratedin the syringe and the bubbles were homogenised in the aqueous phase byagitation. Two different populations of microbubbles i.e. large-sizedand medium-sized were obtained by repeated decantations over shortperiods of time, the large bubbles being collected after only 10-15 minof decantation and the medium sized bubbles being collected after 30-45min. These decantations were repeated 10 times in order to obtain narrowbubble size distributions for the two types of populations and toeliminate all phospholipids which were not associated with themicrobubbles. All phases containing large bubbles were pooled("large-sized bubbles"). Similarly the fractions containing medium sizedbubbles were combined ("medium-sized bubbles"). Aliquots of the twobubble populations were lyophilised and then analysed by HPLC in orderto determine the amount of phospholipids present in each fraction. Thelarge-sized bubble fraction contained 2.5×10⁷ bubbles per ml with amedian diameter in number of 11.3 μm and 13.7 μg phospholipids per 10⁷bubbles. This result is in excellent agreement with the theoreticalamount, 11.5 μg per 10⁷ bubbles, calculated assuming a monolayer ofphospholipids around each bubble and a surface of 50Å per phospholipidmolecule. The medium-sized bubble fraction contained 8.8×10⁸ bubbles perml with a median diameter in number of 3.1 μm and 1.6 μg phospholipidsper 10⁷ bubbles. The latter value is again in excellent agreement withthe theoretical amount, 1.35 μg per 10⁷ bubbles. These results furtherindicate that the stability of the microbubble suspensions hereindisclosed is most probably due to formation of phospholipid monolayersaround the microbubbles.

We claim:
 1. An injectable suspension for ultrasonic echographycomprising a carrier liquid containing at least 10⁷ microbubbles permilliliter and at least one saturated phospholipid, wherein theconcentration of the phospholipid(s) is below 0.01% by weight.
 2. Theinjectable suspension of claim 1, in which the concentration ofmicrobubbles per milliliter is between 10⁸ and 10¹⁰.
 3. The injectablesuspension of claim 1, in which the concentration of phospholipids isabove 0.00013% wt.
 4. The injectable suspension of claim 1, in which theliquid carrier further comprises a stabilizer selected from the groupconsisting of water soluble poly- and oligosaccharides, sugars andhydrophilic polymers.
 5. The injectable suspension according to claim 4,wherein a hydrophilic polymer is a polyethylene glycol.
 6. Theinjectable suspension of claim 1, in which the phospholipids are atleast partially in lamellar or laminar form and are selected from thegroup consisting of lecithins such as phosphatidic acid,phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine,phosphatidylglycerol, phosphatidylinositol, cardiolipin andsphingomyelin.
 7. The injectable suspension of claim 4, furthercontaining a substance affecting the properties of phospholipid selectedfrom the group consisting of phosphatidylglycerol, phosphatidic acid,dicetylphosphate, cholesterol, ergosterol, phytosterol, sitosterol,lanosterol, tocopherol, propylgallate, ascorbyl palmitate and butylatedhydroxytoluene.
 8. The injectable suspension of claim 1, in which thephospholipid is in the form of powders obtained by freeze-drying orspray-drying.
 9. The injectable suspension of claim 1, containing about10⁸⁻ 10⁹ microbubbles per milliliter with the microbubble size between0.5-10 μm showing little or no variation under storage.
 10. Theinjectable suspension of claim 1, in which the liquid carrier furthercomprises up to 50% by weight non-laminar surfactants selected from thegroup consisting of fatty acids, esters and ethers of fatty acids andalcohols with polyols such as polyalkalene glycols, polyalkylenatedsugars and other carbohydrates, and polyalkylenated glycerol.
 11. Theinjectable suspension of claim 1, in which the microbubbles are filledwith SF₆, CF₄, freons or air.